Conventional approaches to optical detection of chemical reactions using surface plasmon resonance typically relies upon a measurement of the optical intensity of the laser light absorbed by a surface excitation in a thin metal layer. A surface plasmon is a localized oscillation of free electrons on the metal surface. The light absorption characteristics of the surface plasmon resonance depend sensitively on the dielectric constant near the metal surface. A chemical reaction occurring at the metal surface modifies the local dielectric environment resulting in a change in the absorption characteristics of the resonance. For further details, see for example, Duyne et al., Journal of the American Chemical Society 2002, 124, 10596–10604.
Present techniques rely on measuring the fraction of incident optical power that is absorbed by the surface plasmon resonance. High sensitivity measurements are typically difficult to make using direct intensity detection, see for example, U.S. Pat. No. 5,506,685. In particular, measurements of optical intensity are directly sensitive to laser intensity noise. Near shot-noise limited measurements may be made by monitoring the phase of the absorbed light as described in Bjorkland et al., Optical Letters, 5, 15, 190 and U.S. Pat. No. 4,297,035, for example. These phase techniques typically require phase modulation of the incident optical field and are typically complex and costly.